I know it was stated in the manga and anime already but i can't remember why!
What did Kurama and Hashimara have I'm common that made this to happen! !??
Narutos cloak was filled with life-force when he first used his Kcm and it made ya matos mokuton grow
Zetsu is a plant ninja made with hashis DNA reacts to the life-force
The life force present in Kurama's chakra causes the wood style to thrive. Because Zetsu is a molecular clone of Hashirama, Kurama's chakra has the sane effect, causing the wood style genes to thrive and sprout.